A Surgical Semiconductor Laser with More Precision and Power

October 29, 1996

By making some simple modifications to semiconductor lasers,
a pair of University of Rochester researchers has devised a way
to make them perform with the power and precision that laser
surgeons routinely demand. These small, relatively inexpensive
lasers might replace more powerful and costly gas lasers in
treating a variety of medical conditions: removing warts, moles,
unwanted hair, and tumors, for instance, performing gum surgery,
or treating glaucoma.

The work, done by graduate student John Marciante and
Professor of Optics Govind Agrawal, was presented last week at
the annual meeting of the Optical Society of America in
Rochester.

Most laser surgery today is done using more powerful and
expensive gas lasers, since their semiconductor counterparts
usually lack the power necessary for most surgery. The Rochester
work cleans up the beam produced by a semiconductor laser,
creating a sharply focused beam with more power and precision
than the beams produced by most such lasers.

"This sounds like a very useful technology for surgery,"
says Raymond Lanzafame, director of Rochester General Hospital's
Laser Center and a practitioner of laser surgery. "The boost in
power provided by this technology could make a single
semiconductor laser an option for many procedures where other
types of lasers are currently used. If you've got a high-quality
beam and a high power output, this offers a convenient
alternative for use in the operating room."

The useful power output of semiconductor lasers is limited
by the tendency of their beams to fragment into a number of
parallel but weaker beams. This doesn't pose a problem for
applications that don't require much power, such as laser
printers or supermarket scanners, but it limits their use in
surgery, where higher power and precision are critical. The new
laser produces a unified beam no wider than a grain of sand, with
the power efficiently packaged in the center. Rochester
researchers believe the new design makes possible semiconductor
lasers with 6 to 12 watts of power, two to four times as powerful
as current devices. Currently the only way for semiconductor
lasers to produce such power is for several to be used in tandem,
but this results in poor beam quality.

"Traditionally, with higher power you lose the ability to
focus a semiconductor laser on its target," Marciante says.
"Rather than a single strong beam, you get three to five weaker
beams, greatly diminishing the laser's power and performance."

To compensate for this breakdown of semiconductor laser
beams -- known as "filamentation" -- Agrawal and Marciante
propose inserting two extra layers of slightly modified
semiconducting material on either side of the active layer of
gallium arsenide, where the beam is formed.

"It's long been known that in filamentation, you get a
'positive' bending in the beams of high-powered semiconductor
lasers, but nobody has been able to successfully counteract that
with a 'negative' bending before," Marciante says. "The layers
we're inserting induce negative bending, resulting in a net bend
of zero for the laser beam and keeping it sharply focused."

While a prototype has yet to be built, Agrawal says that the
new laser performed well in computer simulations. He adds that
since the inserted layers are made of the same semiconducting
materials used in the other layers of the laser -- with a small
amount of aluminum added to give them slightly different
properties -- the new laser shouldn't be significantly costlier
or more difficult to produce than current semiconductor lasers.

Lanzafame, who was not involved in this research, says that
semiconductor lasers offer several advantages over the gas lasers
more widely used in surgery. They're more durable and more user-
friendly, he says, since they have no gas tanks to replace or
mirrors to care for. Semiconductor lasers are also more efficient
than gas lasers: A gas laser can take 5,000 watts of power to run
a one-watt laser, but a semiconductor laser requires only five
watts to produce the same power. Since they can be mass-produced,
semiconductor lasers usually cost only a few thousand dollars,
compared to tens of thousands of dollars for a comparable gas
laser.

Semiconductor lasers also have an advantage over gas lasers
in portability: gas lasers are often immovable refrigerator-sized
units. By contrast, a semiconductor laser is the size of a coarse
grain of salt. Even when a power source is added, a high-power
semiconductor laser apparatus is still no larger than a fist.

The technology may also be applied in the area of
information storage; for instance, the new laser's more precise
focus might permit the storage of more data on CD-ROMs.
Semiconductor lasers are also used in telecommunications and in
communications between satellites.

The work, funded by the University and the U.S. Air Force's
Phillips Laboratory, which employs Marciante, was published in
the July 29 issue of Applied Physics Letters. Agrawal and
Marciante are now considering corporate partners to help build
the new laser.

About the University of Rochester
The University of Rochester (www.rochester.edu) is one of the nation's leading private universities. Located in Rochester, N.Y., the University gives students exceptional opportunities for interdisciplinary study and close collaboration with faculty through its unique cluster-based curriculum. Its College, School of Arts and Sciences, and Hajim School of Engineering and Applied Sciences are complemented by its Eastman School of Music, Simon School of Business, Warner School of Education, Laboratory for Laser Energetics, School of Medicine and Dentistry, School of Nursing, Eastman Institute for Oral Health, and the Memorial Art Gallery.